This invention relates to a method of densifying fumed metal oxides and to further processing of such densified oxides.
A "fumed" metal oxide is a finely divided material, such as silica, that may be prepared by either a vapor phase oxidation or a combustion process. In a typical vapor phase process, vapors of a metal compound, such as silicon tetrachloride (SiCl.sub.4), are thermally converted in the presence of moisture to the corresponding oxide, e.g. silica. In a combustion process, a combustible metal compound, such as a silicon halide or halosilane, is burned to produce the oxide. The former process is described, for example, in U.S. Pats. No. 2,272,342 granted Feb. 10, 1942 to J. F. Hyde, and No. 2,268,589 granted Jan. 6, 1942 to J. A. Heany. The latter process is described, for example, in U.S. Pat. No. 2,823,882, granted Feb. 18, 1958 to O. Saladin et al.
The oxides thus obtained are customarily finely divided, low density, fluffy materials characteristically having an average particle size in the range of 10 to 50 millimicrons and a surface area in the range of 100 to 400 m.sup.2 /gram. Typically, the particles are agglomerated in clusters reminiscent of grape clusters. Such oxide materials are widely used in paints, rubber products, and other organic materials.
It has been recognized that such fumed oxides are highly active, and hence may be consolidated at lower than normal temperatures. In particular, glass forming oxides such as silica can be converted to solid glass bodies below normal melting temperatures. Accordingly, fumed metal oxides, produced by vapor phase or combustion processes, provide a convenient means of obtaining high purity oxide products, such as fused silica glass, by starting with high purity materials. Thus, the purity of the end product may be made dependent on the purity of the materials processed.
For at least one application, high purity glasses may be defined as containing no more than ten parts per million (ppm) alkali metal ions and less than one ppm transition metal ions. This application for high purity glasses occurs in the production of melting crucibles used by the semiconductor industry, in particular, high purity silica crucibles used for melting and drawing silicon. Glasses having a high purity level are also required in a variety of other products, such as antenna windows and radomes. Another area of interest, from a refractory rather than a purity standpoint, is cores for metal castings.
It would therefore be desirable to use fumed oxides in producing such varied high purity products. However, it is difficult to compress these oxides by ordinary pressing or pelletizing techniques to form homogeneous green bodies that may either be vitrified to a body of clear glass or consolidated to a dense ceramic body. Further, in their characteristic low density form, they do not lend themselves to mixing with the vehicles normally used in slip casting. It is therefore a primary purpose of this invention to circumvent these problems by providing a novel method of densifying fumed metal oxides, especially silica, whereby the utility of such oxides in the production of glass and ceramic products is greatly enhanced.